Handlebars.js

Handlebars.js is an extension to the Mustache templating language created by Chris Wanstrath. Handlebars.js and Mustache are both logicless templating languages that keep the view and the code separated like we all know they should be.

Checkout the official Handlebars docs site at http://www.handlebarsjs.com.

Disqus

In Disqus we use Handlebars.js as a main templating language. But we had to extend it a little bit for our own needs. Please see section Differences between Handlebars.js and Disqus-Handlebars below.

Installing

Installing Handlebars is easy. Simply download the package from GitHub and add it to your web pages (you should usually use the most recent version).

Usage

In general, the syntax of Handlebars.js templates is a superset of Mustache templates. For basic syntax, check out the Mustache manpage.

Once you have a template, use the Handlebars.compile method to compile the template into a function. The generated function takes a context argument, which will be used to render the template.

var source = "<p>Hello, my name is {{name}}. I am from {{hometown}}. I have " +
             "{{kids.length}} kids:</p>" +
             "<ul>{{#kids}}<li>{{name}} is {{age}}</li>{{/kids}}</ul>";
var template = Handlebars.compile(source);

var data = { "name": "Alan", "hometown": "Somewhere, TX",
             "kids": [{"name": "Jimmy", "age": "12"}, {"name": "Sally", "age": "4"}]};
var result = template(data);

// Would render:
// <p>Hello, my name is Alan. I am from Somewhere, TX. I have 2 kids:</p>
// <ul>
//   <li>Jimmy is 12</li>
//   <li>Sally is 4</li>
// </ul>

Registering Helpers

You can register helpers that Handlebars will use when evaluating your template. Here's an example, which assumes that your objects have a URL embedded in them, as well as the text for a link:

Handlebars.registerHelper('link_to', function(context) {
  return "<a href='" + context.url + "'>" + context.body + "</a>";
});

var context = { posts: [{url: "/hello-world", body: "Hello World!"}] };
var source = "<ul>{{#posts}}<li>{{{link_to this}}}</li>{{/posts}}</ul>"

var template = Handlebars.compile(source);
template(context);

// Would render:
//
// <ul>
//   <li><a href='/hello-world'>Hello World!</a></li>
// </ul>

Escaping

By default, the {{expression}} syntax will escape its contents. This helps to protect you against accidental XSS problems caused by malicious data passed from the server as JSON.

To explicitly not escape the contents, use the triple-mustache ({{{}}}). You have seen this used in the above example.

Differences between Handlebars.js and Disqus-Handlebars

Although Handlebars.js philosophy is all about logic-less templates, we need some additional functionality in the template language. This allowed us to migrate from our home-grown template language (named DTPL) more easily.

if statements

You can write complex if statements similar to how you do this in plain javascript:

{{#if post.isDeleted || post.isMinimized}} minimized {{/if}}

or

{{#if post.author.id && post.author.id !== "0"}}

Please note that operators like && and !== are separated by spaces. This is a requirement in current version of Disqus-Handlebars. Also you're not allowed to use parenthesis and build very complex if statements. It is recommended to use identity === over equality == operator in your templates.

ifIn helper

The helper's intent is to allow to check whether the value is present in an array. First parameter should be the value (or variable), followed by an array. For example, if we want to see whether string twitter is present in user.connections array, we would write the following:

{{#ifIn 'twitter' user.connections}} do something here {{/ifIn}}

There is an opposite operator ifNotIn for your convenience.

Regular Expressions

ifRegex allows to test any string against some regex expression.

{{#ifRegex '^\S+@\S+\.\S+$' user.email}} do something here {{/ifRegex}}

Translations

All text content should be marked for translation in handlebars templates. Simply pass string into t helper and the string will be replaced with translation when necessary:

{{t "This comment has no content."}}

In some cases you may want to use placeholders in text:

{{t "%(numPosts)s comments" numPosts = count}}

Where numPosts is a named placeholder and count is some variable/property from current context.

You can even insert html from other partial into placeholder:

<h2>{{t "Also on %(forumName)s" forumName = formatForumName(thread.forum) }}</h2>
...
{{#partial 'formatForumName'}}
    <strong>{{name}}</strong>
{{/partial}}

Where forumName is a named placeholder, formatForumName is the partial name and thread.forum is the data context for formatForumName partial. Let's assume that our forum has name My cat can't sleep well. The resulting html (in English) will look like this:

<h2>Also on <strong>My cat can't sleep well.</strong></h2>

You're not limited to use just single placeholder:

{{t "%(user)s posted a %(comment)s in %(thread)s" user = activityUser(activity.object.author) comment = activityComment(activity.object) thread = activityThread(activity.object)}}

Please note that equality sign = is separated by spaces. This is the requirement (and limitation) at the moment.

Scoping and rendering context

Usually templates are rendered based on some JSON data (context). In Disqus we provide two contexts for each template - local and global ones. While local context contains data specific to your current template (or partial), global context consists in variables which are common accross all templates. For example, global context may contain indicator whther user is authenticated or not, available width on a page and so on. Variables from local context should be resolved in usual way (see Handlebars documentation). Varialbes from global context could be accessed via helper g:

<img data-src="{{g 'urls.media'}}/images/noavatar92.png"/>

Partials

In Handlebars partials should be in separate files/templates by default. This is not convenient when you have several groups of small partials. Therefore we introduced new helper named partial which allows you to define several partials in the same file:

{{#partial 'learnMore'}}
  <a href="http://help.disqus.com/customer/portal/articles/123456" target="_blank">{{t "Learn more"}}</a>
{{/partial}}

{{#partial 'feedback'}}
  <a href="https://www.surveymonkey.com/s/ABCD" target="_blank">{{t "give us feedback"}}</a>
{{/partial}}

Iterating thorugh loops

Default each helper remains almost untouched. In addition to standard @index variable, new variable named @length is accessible in any each loop. @length equal to the length of an array you iterating through.

html helper

Output html without characters escpaing (similar to Handlebar's {{> operator, but more intuitive)

{{html post.message}}

Debugging and logging

There are two helpers named debug and log, for developers convenience. log will simply output any variable/property into console. debug is the way to put breakpoint while you render the template.

Differences Between Handlebars.js and Mustache

Handlebars.js adds a couple of additional features to make writing templates easier and also changes a tiny detail of how partials work.

Paths

Handlebars.js supports an extended expression syntax that we call paths. Paths are made up of typical expressions and . characters. Expressions allow you to not only display data from the current context, but to display data from contexts that are descendents and ancestors of the current context.

To display data from descendent contexts, use the . character. So, for example, if your data were structured like:

var data = {"person": { "name": "Alan" }, company: {"name": "Rad, Inc." } };

you could display the person's name from the top-level context with the following expression:

{{person.name}}

You can backtrack using ../. For example, if you've already traversed into the person object you could still display the company's name with an expression like {{../company.name}}, so:

{{#person}}{{name}} - {{../company.name}}{{/person}}

would render:

Alan - Rad, Inc.

Strings

When calling a helper, you can pass paths or Strings as parameters. For instance:

Handlebars.registerHelper('link_to', function(title, context) {
  return "<a href='/posts" + context.url + "'>" + title + "!</a>"
});

var context = { posts: [{url: "/hello-world", body: "Hello World!"}] };
var source = '<ul>{{#posts}}<li>{{{link_to "Post" this}}}</li>{{/posts}}</ul>'

var template = Handlebars.compile(source);
template(context);

// Would render:
//
// <ul>
//   <li><a href='/posts/hello-world'>Post!</a></li>
// </ul>

When you pass a String as a parameter to a helper, the literal String gets passed to the helper function.

Block Helpers

Handlebars.js also adds the ability to define block helpers. Block helpers are functions that can be called from anywhere in the template. Here's an example:

var source = "<ul>{{#people}}<li>{{#link}}{{name}}{{/link}}</li>{{/people}}</ul>";
Handlebars.registerHelper('link', function(options) {
  return '<a href="/people/' + this.id + '">' + options.fn(this) + '</a>';
});
var template = Handlebars.compile(source);

var data = { "people": [
    { "name": "Alan", "id": 1 },
    { "name": "Yehuda", "id": 2 }
  ]};
template(data);

// Should render:
// <ul>
//   <li><a href="/people/1">Alan</a></li>
//   <li><a href="/people/2">Yehuda</a></li>
// </ul>

Whenever the block helper is called it is given two parameters, the argument that is passed to the helper, or the current context if no argument is passed and the compiled contents of the block. Inside of the block helper the value of this is the current context, wrapped to include a method named __get__ that helps translate paths into values within the helpers.

Partials

You can register additional templates as partials, which will be used by Handlebars when it encounters a partial ({{> partialName}}). Partials can either be String templates or compiled template functions. Here's an example:

var source = "<ul>{{#people}}<li>{{> link}}</li>{{/people}}</ul>";

Handlebars.registerPartial('link', '<a href="/people/{{id}}">{{name}}</a>')
var template = Handlebars.compile(source);

var data = { "people": [
    { "name": "Alan", "id": 1 },
    { "name": "Yehuda", "id": 2 }
  ]};

template(data);

// Should render:
// <ul>
//   <li><a href="/people/1">Alan</a></li>
//   <li><a href="/people/2">Yehuda</a></li>
// </ul>

Comments

You can add comments to your templates with the following syntax:

{{! This is a comment }}

You can also use real html comments if you want them to end up in the output.

<div>
    {{! This comment will not end up in the output }}
    <!-- This comment will show up in the output -->
</div>

Precompiling Templates

Handlebars allows templates to be precompiled and included as javascript code rather than the handlebars template allowing for faster startup time.

Installation

The precompiler script may be installed via npm using the npm install -g handlebars command.

Usage

Precompile handlebar templates.
Usage: handlebars template...

Options:
  -a, --amd        Create an AMD format function (allows loading with RequireJS)         [boolean]
  -f, --output     Output File                                                           [string]
  -k, --known      Known helpers                                                         [string]
  -o, --knownOnly  Known helpers only                                                    [boolean]
  -m, --min        Minimize output                                                       [boolean]
  -s, --simple     Output template function only.                                        [boolean]
  -r, --root       Template root. Base value that will be stripped from template names.  [string]

If using the precompiler's normal mode, the resulting templates will be stored to the Handlebars.templates object using the relative template name sans the extension. These templates may be executed in the same manner as templates.

If using the simple mode the precompiler will generate a single javascript method. To execute this method it must be passed to the using the Handlebars.template method and the resulting object may be as normal.

Optimizations

• Rather than using the full handlebars.js library, implementations that do not need to compile templates at runtime may include handlebars.runtime.js whose min+gzip size is approximately 1k.
• If a helper is known to exist in the target environment they may be defined using the --known name argument may be used to optimize accesses to these helpers for size and speed.
• When all helpers are known in advance the --knownOnly argument may be used to optimize all block helper references.

Performance

In a rough performance test, precompiled Handlebars.js templates (in the original version of Handlebars.js) rendered in about half the time of Mustache templates. It would be a shame if it were any other way, since they were precompiled, but the difference in architecture does have some big performance advantages. Justin Marney, a.k.a. gotascii, confirmed that with an independent test. The rewritten Handlebars (current version) is faster than the old version, and we will have some benchmarks in the near future.

Building

To build handlebars, just run rake release, and you will get two files in the dist directory.

Upgrading

When upgrading from the Handlebars 0.9 series, be aware that the signature for passing custom helpers or partials to templates has changed.

Instead of:

template(context, helpers, partials, [data])

Use:

template(context, {helpers: helpers, partials: partials, data: data})

Known Issues

• Handlebars.js can be cryptic when there's an error while rendering.
• Using a variable, helper, or partial named class causes errors in IE browsers. (Instead, use className)

Handlebars in the Wild

• jblotus created http://tryhandlebarsjs.com for anyone who would like to try out Handlebars.js in their browser.
• Don Park wrote an Express.js view engine adapter for Handlebars.js called hbs.
• sammy.js by Aaron Quint, a.k.a. quirkey, supports Handlebars.js as one of its template plugins.
• SproutCore uses Handlebars.js as its main templating engine, extending it with automatic data binding support.
• Ember.js makes Handlebars.js the primary way to structure your views, also with automatic data binding support.
• Les Hill (@leshill) wrote a Rails Asset Pipeline gem named handlebars_assets.

Helping Out

To build Handlebars.js you'll need a few things installed.

• Node.js
• Jison, for building the compiler - npm install jison
• Ruby
• therubyracer, for running tests - gem install therubyracer
• rspec, for running tests - gem install rspec

There's a Gemfile in the repo, so you can run bundle to install rspec and therubyracer if you've got bundler installed.

To build Handlebars.js from scratch, you'll want to run rake compile in the root of the project. That will build Handlebars and output the results to the dist/ folder. To run tests, run rake spec. You can also run our set of benchmarks with rake bench.

If you notice any problems, please report them to the GitHub issue tracker at http://github.com/wycats/handlebars.js/issues. Feel free to contact commondream or wycats through GitHub with any other questions or feature requests. To submit changes fork the project and send a pull request.

License

Handlebars.js is released under the MIT license.